Did your prototype use the Vishay devices?
Without seeing your schematic it is impossible to be sure, but is it possible your current limiting resistor for the zener is not allowing enough current to the zener to obtain the 50 mA test current ?
According to the data sheet, 15.6V is the tolerance maximum with a test current of 50mA, I'm only passing half that.
The other issue you have to be aware of is counterfeit or sub-spec components being used byunscrupulous companies - this is a big problem and its also possible you have been a victim.
Therefore you can't say the part is out of specification. Passing lees that the test current will mean that the zener voltage will be greater than it would be if you were to be passing the test current.
If you weant an accurate 15v ref shunt regulator you'd be much better usiing eg a TL431if you want it spot-on use SOT or a potentiometer.regardsAllan